WASHINGTON (AP) - Republicans and Democrats put goodwill to the test as Congress returns this week to potentially incendiary fights over nominations, unresolved disputes over student loans and the farm bill, and the uncertainty of whether lawmakers have the political will to rewrite the nation's immigration laws.

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The rare cooperation on display in the Senate last month with passage of a bipartisan immigration bill could be wiped out immediately if Majority Leader Harry Reid, frustrated with minority Republicans' delaying tactics on judges and nominations, tries to change the Senate rules by scrapping the three-fifths majority for a simple majority.

Republican leader Mitch McConnell has indicated it is a decision Reid could regret if the GOP seizes control in next year's elections.

"Once the Senate definitively breaks the rules to change the rules, the pressure to respond in kind will be irresistible to future majorities," McConnell said last month, looking ahead to 2014 when Democrats have to defend 21 seats to the GOP's 14.

McConnell envisioned a long list of reversals from the Democratic agenda, from repealing President Barack Obama's signature health care law to shipping radioactive nuclear waste to Yucca Mountain in Reid's home state of Nevada.

Recently elected Democrats have clamored for changes in Senate rules as Obama has faced Republican resistance to his nominations. Two Cabinet-rank choices - Tom Perez as Labor secretary and Gina McCarthy to head the Environmental Protection Agency - could be approved by the Senate this month after a loud debate over administration policies. The GOP also has challenged Obama's three judicial nominees to the U.S. Court of Appeals for the District of Columbia Circuit.

Reid had served notice in April that the Democratic majority could change the Senate rules on "any given day," and he was willing to do so if necessary.